1. Field of the Invention
The present invention relates generally to an apparatus for magnetically reproducing a video signal, and more particularly, to a video signal reproducing apparatus having a rotary magnetic head making one turn thereof during one field period of a video signal treated thereby in contact with a magnetic tape in a normal operational mode thereof.
2. Description of the Prior Art
There has been proposed a video tape recorder (VTR) of the helical scan type having a rotary magnetic head which is rotated to make each one turn for scanning a magnetic tape during one field period of a video signal recorded thereby on or reproduced thereby from the magnetic tape in a recording or normal reproducing mode thereof. A typical example of a head drum arrangement employed in such a video tape recorder as mentioned above is shown in FIG. 1. In this example, a head drum 1 compose of a rotatable upper cylinder and a stationary lower cylinder (not shown), and a rotary magnetic head 2 is fixed to the bottom of the rotatable upper cylinder which faces to the top of the stationary lower cylinder. A magnetic tape 3 which is guided by a couple of guide pins 4 and 5 is wound around the head drum 1 to be obliquely scanned by the rotary magnetic head 2 when the latter is rotated in accordance with the rotation of the rotatable upper cylinder. In the recording or normal reproducing mode of the video tape recorder, the rotatable upper cylinder to which the rotary magnetic head 2 is fixed is rotated at such a rotating speed as to make each one turn during one field period of a video signal treated by the rotary magnetic head 2 in the direction shown with an arrow 6, and the magnetic tape 3 is transported in the direction shown with an arrow 7. Accordingly, in the recording mode, each segment of one field period of the video signal is recorded in a slant track formed on the magnetic tape 3 by one turn of the rotary magnetic head 2 except a part of the segment in a vertical blanking period of the video signal. Such a part of the segment which is not recorded on the magnetic tape 3 is caused during a short period in which the rotary magnetic head 2 does not come into contact with the magnetic tape 3.
When a so-called "trick" reproducing mode, such as, a fast-forward reproducing mode in which the magnetic tape 3 is advanced at a speed N (N is an integer) times as high as normal speed used for the recording or a normal reproducing mode, a still reproducing mode for obtaining a still reproduced picture, a reverse reproducing mode in which the magnetic tape 3 is reversed at the normal speed or a fast-reverse reproducing mode in which the magnetic tape 3 is reversed at the speed N times as high as the normal speed is taken in such a video tape recorder of the helical scan type as mentioned above, the scanning trace of the rotary magnetic head 2 on the magnetic tape 3 is not coincident with each of slant tracks formed on the magnetic tape 3 in the normal recording mode, as shown in FIG. 2. In FIG. 2, the magnetic tape 3 is provided with a plurality of slant tracks 9 formed in the recording mode of the video tape recorder wherein the magnetic tape 3 is advanced at the normal speed in the direction shown with the arrow 7 and the rotary magnetic head 2 is moved in the direction shown with an arrow 8. The scanning trace of the rotary magnetic head 2 in the normal reproduciing mode wherein the magnetic tape 3 is advanced at the normal speed is coincident with each slant track 9 as indicated by an arrow +1, but each of the scanning traces of the rotary magnetic head 2 in the "trick" reproducing modes intersects the slant tracks 9 in such a manner as indicated by an arrow +2 in a twice fast-forward mode wherein the magnetic tape 3 is advanced at a speed twice as high as the normal speed, by an arrow +3 in a three times fast-forward mode wherein the magnetic tape 3 is advanced at a speed three times as high as the normal speed, by an arrow 0 in the still reproducing mode, by an arrow -1 in the reverse reproducing mode, by an arrow -2 in a twice fast-reverse reproducing mode wherein the magnetic tape 3 is reversed at a speed twice as high as the normal speed and by an arrow -3 in a three times fast-reverse reproducing mode wherein the magnetic tape 3 is reversed at a speed three times as high as the normal speed.
In view of this, for the purpose of causing the scanning trace of a rotary magnetic head in the "trick" reproducing mode of a video tape recorder of the helical scan type to be coincident with each of selected slant tracks on a magnetic tape so that a reproduced video signal with reduced noise components, such as a video signal reproduced in the normal reproducing mode, is obtained, it has been proposed to mount the rotary magnetic head on a compound piezoelectric plate element which is composed of a couple of piezoelectric plate members bonded to each other with the common polarizing direction and fixed at one end thereof to a rotatable upper cylinder of a head drum so that the rotary magnetic head is rotated together with the rotatable upper cylinder and a position of the rotary magnetic head in the direction of the axis for rotation thereof is shifted in response to a voltage applied to the compound piezoelectric plate element and to supply to the compound piezoelectric plate element with a saw-toothed waveform voltage having a period corresponding to the period in which the rotary magnetic head makes one turn thereof in the "trick" reproducing mode, as disclosed in U.S. Pat. Nos. 4,287,538 or 4,319,289. In this proposal, the saw-toothed waveform voltage is selected to have the level increasing (or decreasing) with a first inclination determined in response to the direction and amount of the positional deviation of the scanning trace of the rotary magnetic head from each slant track on the magnetic tape, which is to be caused when any saw-toothed waveform voltage is not supplied to the compound piezoelectric plate element, in the period in which the rotary magnetic head scans the magnetic tape along the slant track from the beginning end to the terminal end thereof, and then, decreasing (or increasing) with a second inclination in the period wherein the rotary magnetic head is not in contact with the magnetic tape.
In the case where this head control mechanism is employed in the video tape recorder of the helical scan type having such a head drum arrangement as shown in FIG. 1, since the rotary magnetic head must jump to a beginning position for slant tracks, such as indicated with X in FIG. 2, from a terminal position for slant tracks, such as indicated with Y in FIG. 2, on the magnetic tape in a period shorter than the vertical blanking period of the video signal in order to shift from a certain selected slant track to the next selected slant track, the saw-toothed waveform voltage supplied to the compound piezoelectric plate element is required to have a steep level inclination in the short period in which the rotary magnetic head jumps to the beginning position for slant tracks from the terminal position for slant tracks on the magnetic tape. Accordingly, as shown in FIG. 3A by way of example, a saw-toothed waveform voltage Es with a cyclical period corresponding to a field period of the video signal, which has a gently increasing slope during a period Px in which the rotary magnetic head scans the magnetic tape from the beginning position for slant tracks to the terminal position for slant tracks thereon and a steeply decreasing slope duriing a period Py in which the rotary magnetic head jumps from the terminal position for slant tracks to the beginning position for slant tracks on the magnetic tape is supplied to the compound piezoelectric plate element.
In such a case, however, as shown in FIG. 3B, swinging movements Ry and Rx of the compound piezoelectric plate element with a specific resonant frequency result from the changes of the slope of the saw-toothed waveform voltage Es occuring at a point t.sub.1 at which the rotary magnetic head leaves the terminal position for slant tracks on the magnetic tape and a time point t.sub.2 at which the rotary magnetic head enters the beginning position for slant tracks on the magnetic tape, respectively, and as a result of these swinging movements Ry and Rx, a residual swinging movement Rz of the compound piezoelectric plate element is caused at the beginning portion of the period Px as shown by means of a positional variation Hp of the rotary magnetic head in the direction of the axis for rotation thereof in FIG. 3C, so that the rotary magnetic head can not trace correctly the slant track on the rotary magnetic head. Such swinging movements occuring in connection with the rotary magnetic head is disclosed in detail in U.S. Pat. Nos. 4,163,994 or 4,172,265.
Further, in the case where, for the purpose of causing the rotary magnetic head to trace correctly slant tracks formed with inferior linearity on the magnetic tape, a sine-curve waveform voltage having a frequency higher than the field frequency of the video signal, for example, 720 Hz is also supplied to the compound piezoelectric plate element so as to cause the rotary magnetic head to vibrate in the direction of the axis for rotation thereof in accordance with a so-called "wobbling" method, such as disclosed, for example, in U.S. Pat. No. 4,404,605, a more undesirable movement of the rotary magnetic head may be caused by the beat arising between the vibration generated by the sine-curve waveform voltage and the swinging movement aforementioned.
The magnitude of the residual swinging movement Rz of the compound piezoelectric plate element at the beginning portion of the period Px in which the rotary magnetic head scans the magnetic tape from the beginning position for slant tracks to the terminal position for slant tracks thereon, which is indicated with A in FIG. 3A, is varied remarkable in response to variations of the duration T of the period Py in which the rotary magnetic head jumps from the terminal position for slant tracks to the beginning position for slant tracks on the magnetic tape though it is also varies slightly in response to variations in amplitude of the saw-toothed waveform voltae Es. FIG. 4 shows an example of the relation between the magnitude A of the residual swinging movement Rz and the duration T of the period Py obtained practically under the condition where the frequency of the saw-toothed waveform voltage Es is 60 Hz, namely, the field frequency of the video signal, the amplitude of the saw-toothed waveform voltage Es is 250 volts and the specific resonant frequency of the compound piezoelectric plate element is 1425 Hz (the specific resonant period is 0.70 milliseconds). In FIG. 4, the magnitude A is referred to as a voltage value which is to be applied to the compound piezoelectric plate element for making the same have a deviation of the magnitude A in the static condition.
As apparent from the relation shown in FIG. 4, because the duration T of the period Py is long or the saw-toothed waveform voltage Es has a gentle slope during the period Py it does not always follow that the magnitude A of the residual swinging movement Rz is reduced, and in the condition wherein the duration T of the period Py is shorter than the vertical blanking period of the video signal, the magnitude A of the residual swinging movement Rz is minimized when the duration T of the period Py coincides substantially with the specific resonant period of the compound piezoelectric plate element. In the case of this example, the specific resonant period of the compound piezoelectric plate element is 0.70 milliseconds and it has been proven that the magnitude A of the residual swinging movement Rz is minimized at an equivalent voltage value of about 75 millivolts when the duration T of the period Py is about 0.72 milliseconds. This conclusion is explained as follows. In the case where the duration T of the period Py coincides substantially with the specific resonant period of the compound piezoelectric plate element, the swinging movement Ry of the compound piezoelectric plate element resulting from the change of the slope of the saw-toothed waveform voltage Es occurring at the time point t.sub.1 at which the rotary magnetic head leaves the terminal position for slant tracks on the magnetic tape, as shown in FIG. 3B, is opposite in phase to the swinging movement Rx of the compound piezoelectric plate element resulting from the change of the slope of the saw-toothed waveform voltage Es occurring at the time point t.sub.2 at which the rotary magnetic head enters the beginning position for slant tracks on the magnetic tape, as shown also in FIG. 3B, so that both swinging movements Ry and Rx partially cancel out each other.
In view of this, it is understood that the residual swinging movement Rz of the compound piezoelectric plate element can be minimized by setting the duration T of the period Py to be coincident with the specific resonant period of the compound piezoelectric plate element. However, in such a case, as apparent from the relation shown in FIG. 4, the minimized residual swinging movement Rz is still relatively large, because the swinging movement Ry of the compound piezoelectric plate element resulting from the change of the slope of the saw-toothed waveform voltage Es occuring at the time point t.sub.1 decreases in the period Px to be insufficient for cancelling satisfactorily the swinging movement Rx of the compound piezoelectric plate element resulting from the change of the slope of the saw-toothed waveform voltage Es occuring at the time point t.sub.2.